Protective systems and apparatus therefor

ABSTRACT

A protective system capable of being monitored for day and night use comprising a single electrical loop including a plurality of series-connected contacts arranged to be actuated by an intruder and a relay connected in the loop and arranged to be rendered operative to actuate associated contacts connected in an alarm circuit to indicate the presence of the intruder, selected ones of the contacts having diodes connected across the contacts such that the system can operate in accordance with two different modes. For day use, the current flow through the diodes is such that no alarm indication is provided if a series-connected contact spanned by a diode is actuated, while for night use, a switching unit is provided to reverse the direction of current flow through the loop to cause the relay to be rendered operative if any series-connected contact is actuated.

United States Patent [72] Inventor Goodwin A. G. lve I Farm Cottage,High Road, Old Chipstead, In the County of Surrey, England [21] Appl.No. 818,860 [22] Filed Apr. 24, 1969 [45] Patented Dec. 7, 1971 [32]Priority Apr. 29, 1968 [33] Great Britain 20,320/68 [54] PROTECTIVESYSTEMS AND APPARATUS THEREFOR 16 Claims, 6 Drawing Figs.

[52] US. Cl. 340/276, 340/274, 340/409 [51] Int. Cl ..G08b 13/08 [50]Field 01 Search 340/274, 276, 409

[56] References Cited UNITED STATES PATENTS 2,944,252 7/1960 Muehter340/276 2,971,186 2/1961 Ripepi 340/276 3,010,100 11/1961 Muehter340/276 Primary Examiner-John W. Caldwell Assistant Examiner-J, MichaelBobbitt Anomey-Stevens, Davis, Miller and Mosher ABSTRACT: A protectivesystem capable of being monitored for day and night use comprising asingle electrical loop including a plurality of series-connectedcontacts arranged to be actuated by an intruder and a relay connected inthe loop and arranged to be rendered operative to actuate associatedcontacts connected in an alarm circuit to indicate the presence of theintruder, selected ones of the contacts having diodes connected acrossthe contacts such that the system can operate in accordance with twodifferent modes. For day use, the current flow through the diodes issuch that no alarm indication is provided if a series-connected contactspanned by a diode is actuated, while for night use. a switching unit isprovided to reverse the direction of current flow through the loop tocause the relay to be rendered operative if any series-connected contactis actuated.

PATENTEB DEB 712. 1 I 3 34 sum 2 OF 3 n V F I M, F; I

r2 r3 02 Rm r0 H 560 II 05 ALA L! 00/! N i 01/2 :4 I as l l u re M2 r4RI s1 04 m ws W m INT/2005K I FIG 3 ALARM Ll/IT J as \IM3 I Ya INVENTOR6001mm! IVE BY 0 f ATfdR/VZVS PROTECTIVE SYSTEMS AND APPARATUS THEREFORThis invention relates to protective systems and apparatus therefor suchas intruder protection, fire and bandit warning systems includingintruder detection and warning devices and fire and bandit warningalarms and detectors capable of providing a warning indication of anyabnormal occurrence such as the entry of an intruder into a building, orthe outbreak of heat, smoke or fire.

Heretofore, with some intruder protection systems, a closed electricalcircuit loop has been provided having wired therein series connected,normally closed contacts associated with doors, windows and the like andarranged to open and close with opening and closing of the doors,windows etc., and parallel-connected, normally open contacts associatedwith floor pressure mats for instance and arranged to close underpressure.

When the intruder protection circuit is operative, a current flowsthrough the series-connected contacts of the loop to effect energizationof an electromagnetic relay until one or more of the series-connectedcontacts is opened in response to the opening of an associated door orwindow by an intruder, following which the relay is deenergized toeffect actuation of associated relay contacts connected in an alarmcircuit such that a warning indication is provided of the presence ofthe intruder. Furthermore, the application of pressure to a floor matdetector will close the associated parallel-connected contacts to bypassor short" the relay winding thereby actuating the relay contacts in thealarm circuit and again providing an indication of the presence of anintruder. It will be appreciated that such a closed loop protectivecircuit provides a warning indication also if a short circuit or opencircuit condition develops in the loop.

Open circuit loops have also been provided having normally open contactsconnected across a battery or the like connected in series with thewinding of a relay, such that closure of a contact in response to abreak-in or similar occurrence effects a short circuit therebydeenergizing the relay winding to provide an indication of the presenceof an intruder.

A disadvantage of the closed loop system is that when such a closed loopis installed in a building, it is necessary to switchoff the systemduring those periods when the building is occupied otherwise the alarmcircuit would be operative to provide a warning indication each time adoor or window was opened or a person closed the contacts of a floor matdetector. However, it is during the switch-off periods that the systemmay be inadvertently or deliberately damaged and such damage mightescape detection until the system is switched on again when allpersonnel have vacated the building.

Such a situation leads to considerable inconvenience requiring as itdoes engineers standing by on call at peak closing hours in order torepair any such damage which might have occurred during the switched-ofiperiod.

A further disadvantage of the present system is that the whole system isswitched off during those periods when the building is occupied whereasa certain degree of protection may be required during those periods atselected points for instance at basement windows, skylights, rear doors,etc. With present systems, in order to achieve such selectiveprotection, a second wiring loop is provided but such an arrangementadds considerably to the cost of the overall installation.

Accordingly, it is an object of the present invention to provide aprotective system and apparatus thereof including a single electricalloop but capable of providing selective protection when required andcapable of being monitored 24 hours a day in order to overcome thedisadvantages previously specified.

A further object of the present invention is to provide a protectivesystem and apparatus therefor including a single electrical loop capableof operating in accordance with two different operative modes.

A still further object of the invention is to provide a twowireprotective loop capable of providing an alarm indication whenever thetwo wires are wholly or partially short circuited or one or other orboth open circuited.

According to the invention there is provided a protective systemcomprising a single electrical loop including a bistate control devicehaving means associated with an alarm circuit and arranged to beactuated in response to a change in the operative state of said bistatecontrol device, the loop being connected to a voltage source and alsoincluding a plurality of contacts arranged to be actuated whenever anabnormal situation against which protection is required, occurs, and theloop being arranged to operate in accordance with two alternativeoperating modes during one of which the current flowing through the loopis sufficient to allow said bistate control device to operate inaccordance with a first operative state despite actuation of any of saidcontacts, and during the second mode the current through the loop beingsuch as to cause the bistate control device to change to the other ofsaid operative states whenever a contact is actuated.

In one embodiment of the invention there is provided a protective systemincluding a closed electrical loop connected across the winding of arelay having contacts connected in an alarm circuit, the loop includinga plurality of series-connected contacts arranged to be actuated when anabnormal situation occurs, a respective unidirectionalcurrent-conducting device connected across selected ones of saidcontacts such that the system is capable of operating in accordance withone current-conducting mode and switching means for inhibiting currentflow through said unidirectional currentconducting means such that thesystem operates in accordance with a second current-conducting mode.

In a further embodiment of the invention there is provided a protectivesystem as described above wherein an additional relay is included in theloop together with normally open parallel-connected contacts connectedin series with impedance means, the arrangement being such that whenevera parallel-connected contact is closed, the current flow through theadditional relay is increased to an extent sufiicient to change thestate thereof but without effecting any change in the state of saidfirst mentioned relay.

In another embodiment of the invention there is provided a protectiveloop system as described above having a two-wire loop terminated byimpedance means and having a relay means arranged to be actuatedwhenever a whole or partial short circuit or open circuit conditionoccurs in the loop, second relay means operable to provide secondchannel operation and interlocking means associated with the two relaymeans to prevent operation of the second relay means should such a wholeor partial short circuit or open circuit condition occur.

Several embodiments of the invention will now be described by way ofexample only with particular reference to the accompanying drawingswherein:

FIG. I is a circuit diagram of a conventional "closed loop" intruderalarm system;

FIG. 2 is a circuit diagram of a closed loop" intruder alarm systemaccording to the present invention;

FIG. 3 is a circuit diagram of a closed loop" combined intruder andwarning alarm system of the present invention;

FIG. 4 is a schematic circuit diagram of part of an open loop intruderalarm system;

FIG. 5 is a circuit diagram of part of an AC-DC circuit for an intruderalarm system and FIG. 6 is a circuit diagram illustrating how theresponse time of a circuit is utilized to provide an intruder alarmindication.

Referring to FIG. I the closed electrical loop includes a plurality ofnormally closed series-connected contacts CI to C5 which may beassociated with windows, doors, etc., and arranged to be opened when theassociated door or window is opened, and a pair of normally openparallel-connected contacts M1, M2, which may be associated with contactmats or the like each arranged to be closed when pressure is applied tothe respective contact mat. The reference Ws indicates a wire screenprovided for protection against intruders at windows, glass doors, etc.The contacts C1 to C5 are connected in series with a relay R1 andcontacts M1 and M2 are connected across the relay which is normallyenergized when the series-connected contacts C1 to C are closed, asshown, and the parallel-connected contacts M1, M2, are open. The relayRll has associated contacts Rl/l connected in an alarm circuit and thecontacts Rl/l are arranged to be actuated upon deenergization of relayR1 to provide an alarm indication of the presence of an intruder,whenever one or more of the contacts C1 to C5 is or are opened, or oneor more of contacts M1 M2, is or are closed, or the wire screen Ws isbroken.

A battery B completes the loop and has a limiting resistor r, connectedin series therewith in order to protect the battery when a short circuitoccurs.

With such an arrangement it is impossible to monitor the protective loopor provide selective protection for instance when a building protectedby the loop is occupied and when the doors or windows protected by thecontacts C1 to C5 are in use and personnel are liable to tread on thepressure mats protected by contacts M1, M2. Accordingly it is necessaryto switch-off the system during occupation of the building, during whichperiod the loop may be damaged and escape detection, and no selectiveprotection is possible with such a prior art arrangement.

Referring now to the arrangement illustrated in FIG. 2 unidirectionalcurrent-conducting devices such as semiconductor diodes D1, D2, D5, areconnected across the respective series-connected contacts C1, C2, C5,and a further diode D4 is connected in series with parallel-connectedcontact M2. For periods during which the premises protected by the loopare occupied, e.g., during daylight hours, the polarity of the currentflowing through the loop is arranged such that diodes D1, D2 and D5 areforward connected such as to pass current whenever the respectivecontact C1, C2 or C5 is opened, whilst diode D4 is reverse connectedsuch as to inhibit the passage of current when contact M2 is closed.

If any of contacts C1, C2 or C5 are opened such as by opening of thedoors or windows associated with those contacts, the respective diodesD1, D2, D5 will conduct in the forward direction and will permitsufficient current to flow in the loop to retain relay R1 in theenergized condition. Thus the contacts R1 1 are not actuated and noalarm indication is given.

Similarly, if contact M2 is closed by pressure applied to the associatedmat, since the diode D4 is reverse connected, no current will flowtherethrough and no shunting of relay R1 will occur Thus the relay R1will remain energized and no alarm indication will be given.

If the contacts C3 or C4, are opened or screen Ws is broken, since thesecontacts are not shunted by a diode and are effective to provide anindication of the passage of an intruder through the window or doorassociated therewith, the relay R1 will be deenergized to actuatecontacts RH]. and provide an alarm indication of the brealcin. Similarlycontact Ml has no diode in series therewith and if the mat associatedwith contact Ml has pressure applied thereto, the contact M1 will closeto short circuit relay R1 thereby deenergizing the relay and actuatingcontacts R1 /1 to provide an alarm indication.

It will be appreciated that the above arrangement allows the circuit tobe monitored during the period when the building is occupied andprovides protection at selected points of the building.

In order to provide complete protection during those periods when thebuilding is unoccupied a changeover switch (not shown) is providedwhereby the direction of current flow through the loop is reversed.Diodes D1, D2 and D5 will now be reverse-connected and diode D4 will beforward connected. Thus, if contacts C1, C2 or C5 are opened by openingof the door or window protected thereby, the leakage current through thereverse connected diode will be insufficient to maintain energization ofthe winding of relay R 1. Accordingly, contacts Rl/l will be actuated toprovide an alarm indication of the presence of an intruder. Furthermoresince diode D4 is now forward connected, if contact M2 is closed diodeD4 applies a short circuit across the relay winding and thus relay R1 isdeenergized and the alarm indication given. With the above arrangementcomplete protection is available at all detection points and thusselective protection is possible when the current is flowing in onedirection through the loop and complete protection is provided when thedirection of current flow is reversed.

Referring now to the circuit illustrated in FIG. 3 of the drawings, thesame contacts C1, C2, C5 and M2 have been provided with respectivediodes D1, D2, D5, D4, but in addition thereto, normally open detectionor alarm contacts F1, F2, F3, F4, have been connected across the loop inseries with respective resistors r r;,, r r and an additional relay R2has been included in the loop with the winding thereof in series withbattery B in place of resistor r,. The contacts R2/1 associated withrelay R2 may be connected in a fire-warning alarm circuit or in abandit-warning alarm circuit.

The circuit operates in the selective and complete" protection modes asdescribed above for intruder protection and for additional fire orbandit protection. The resistance and ampere-tums of coil R2 are chosensuch that with both relays R1 and R2 in series, the current passingthrough the loop is sufficient to energize relay R1 but insufficient toenergize relay R2. If necessary, the applied voltage V may be increasedto allow for the additional voltage drop across the winding of relay R2.

1f any of the contacts F1 to F4 close in response to the detection ofheat, fire or smoke, or in response to depression of a bandit or hold-upalarm switch or the like, relay R1 will be shunted by the respectiveresistor r to r However, the individual ohmic value of resistors r to ris selected such that the shunting effect on relay R1 is insufficient todeenergize this intruder alarm relay R1. Nevertheless, the current whichnow flows through the resistor is sufficient to effect energization ofrelay R2 thereby actuating contacts R2/l and, in the case of a firealarm system, provide an alarm indication of the existence of the fire,heat or smoke.

A latching contact R1/2 is provided associated with the reset switch S1.As before, closure of contacts M1 or M2 will effect deenergization ofrelay R1, but at the same time, the current through the loop isincreased due to a virtual short circuit caused by closure of contactsM1 or M2. Consequently, the latching contact R1/2 is included in thecircuit to prevent energization of relay winding R2 in response toclosure of contacts M1 or M2, and to prevent a false fire alarm orhold-up alarm from being given. When relay R1 is deenergized, thelatching contact Rl/2 is actuated to open the loop circuit therebyretaining relay R2 in the deenergized state and preventing an alarm frombeing given. The loop will remain in the open circuit condition untilreset switch S1 is operated.

When one of the contacts F1 to F4 close in response to the detection ofheat, fire or smoke or in response to depression of a bandit" orhold-up" alami switch, relay R1 is not affected and remains energizedand the loop circuit remains closed via latching contact R1/2. Anincreased current then flows due to the short circuit initiated byclosure of a contact F1 to F4, and relay R2 is energized to effectactuation of contacts R2/l thereby providing a fire or bandit warningalarm indication.

it will be appreciated that an advantage of the circuit shown in HO. 3is that since the detection" contacts F1 to F4 have no diodes or similarunidirectional current-conducting devices connected in series or inparallel therewith, the contacts are not polarity sensitive, i.e., theyare operative irrespective of the direction of current flow through theloop and thus detection facilities are provided for 24 hours of everyday if neces' sary.

The invention is in no way limited to the closed circuit loop shown inFIGS. 2 and 3 and an open loop is shown in H0. 4 havingparallel-connected contacts M3, M4, connected across the battery 8 andrelay Rl. FOr dual-mode operation, a semiconductor diode D5 is connectedin series with contact M4 and the circuit operates in a similar mannerto that part of the circuit already described with reference to F10. 2and includes contact M2, diode D4 and relay R1.

Furthermore it will be appreciated that the invention has been describedby way of example only and is susceptible of considerable modification.For instance, electrolytic capacitors may be used as the unidirectionalcurrentconducting devices, instead of the semiconductor diodes. Anelectrolytic capacitor will, when connected in circuit with a voltagesupply become charged such as to inhibit the flow of current in thecircuit. If the polarity of the applied voltage is reversed, thecapacitor behaves as an impedance and permits current to flowcontinuously since the capacitor is no longer being charged. Thus, anydevice which changes the magnitude of the current flowing in a circuitwhen a change of polarity occurs, may be used, and the termunidirectional current-conducting device is to be construed to includeany such device.

From the foregoing it will be appreciated that the present inventionprovides a protective system capable of operating in accordance with twodifferent modes which may be termed conveniently day" and night" modesrespectively. However, it is not necessary for the change in mode to bedependent upon a change in the direction of the current flowing througha loop or upon the use of unidirectional current-conducting devices. Thechange in mode may be effected by means of a change in the appliedvoltage or by changing the response time of the circuit, which for thepurposes of the following explanation, includes the relay winding orwindings.

A change in the applied voltage may be effected by a. a reversal ofpolarity, e.g., of the battery B in order to produce a change in thedirection of current flow through the loop as already described;

b. a change in the waveform of the voltage signal, e.g., from an ACvoltage to a DC voltage;

c. a change in the value of the voltage applied to the relay or relaysand d. a change in the response time of the circuit which may beconveniently effected by changing the response time of the relay orrelays.

Referring to (b) above, a circuit for changing from an AC voltage duringthe dayto a DC voltage at night is shown in FIG. 5. The series-connectedcontacts C1, C2, C3, are arranged to be connected in circuit withterminals a, b, of an AC voltage source via switches S2, S3 during thedaytime such as to operate in accordance with one circuit mode, and tobe connected via switches S2, S3, to terminals c, d, of a DC voltagesource for night operation. Contacts C1 and C3 have capacitors C10, C11,shunted thereacross and a parallel-com nected contact M1 is connectedacross the relay R1. An associated relay contact RI/ 1 is normallyclosed and arranged to be opened when the relay R1 is deenergized.

During the day, an AC voltage is applied to the loop and if one of thecontacts C1 or C3 shunted by capacitors C10, C11 respectively, isopened, current continues to flow to maintain the relay energized. Thevalue of the current is determined by the resistance of the capacitor.For night security, the contacts S2, S3 are switched to terminals c, d,respectively. If a contact C1 or C3 opens, the respective capacitorcharges to inhibit the flow of current through the loop and the relay R1is deenergized to actuate the associated relay contacts in theintruderwaming alarm circuit. Similarly, the relay will becomedeenergized when a short circuit occurs.

Referring to paragraph (c) above, any contacts which might be openedduring daytime use are shunted by resistors such as to render theopening of a respective contact ineffective. When a contact is opened,the resistor is switched into circuit and the voltage across theprotective relay decreases, but remains of sufficient magnitude toretain the relay energized.

For night security, the value of the applied voltage is decreased andthe additional resistance produced by the opening of a contact,decreases the relay voltage to a value sufficient to causedeenergization thereof.

Referring to paragraph (d) above, and as shown in FIG. 6, all contactsrequired to remain ineffective during one circuit mode, are of thesingle pole changeover type with the contacts looped as shown at x andy. During the day, if a contact C1 or C3 is opened, a momentary breakonly occurs. During the contact changeover period, the relay R1 is heldenergized by the capacitor C12 across the winding thereof. For nightsecurity purposes, the capacitor is taken out of circuit by actuation ofswitch S3 and a momentary break in the circuit will then deenergize therelay R1 which trips via its associated latching contact Rl/l.

Instead of a battery B, the DC voltage may be derived from a DC powerpack supplied from the AC mains. As previously described for periodsduring which a building is occupied, the applied voltage may beincreased above the nominal voltage to compensate for the voltage dropacross the diodes. The voltage supply during the occupied" periods maybe separate from that employed during unoccupied" periods. The formermay be derived from a DC powerpack and the latter from a battery.Alternatively, for unoccupied periods, the voltage may be derived from apowerpack with standby battery facilities; this facility being lessnecessary during occupied" periods when alarms due to a supply failureare unimportant. Where a battery standby is available, this may mask asupply fault, such as a blown fuse, in which case the battery would beused unnecessarily, thus shortening the working life of the battery. Ifno standby battery is employed during occupied" hours, such faults wouldbe immediately detected.

The diodes connected across the series-connected contacts or connectedin series with the parallel-connected contacts of the loop may bemounted separately from the contacts or incorporated within a switchunit, particularly where the contacts may be of the magnetic reed type.Such a switch unit is described in copending British Pat. applicationNo. 50,385/68 and the switch unit may include a switch and at least oneunidirectional current-conducting device and/or impedance meansconnected in series with or across the contacts of the switch.

The relays employed in the present invention may be of any suitable typesuch as electromagnetic or transistor relays.

For an intruder alarm circuit only such as illustrated in FIG. 2, onlyone relay is used in the protective loop but since this relay isself-resetting, it must be provided with a latching contact, or mayoperate a second relay which latches and completes the alarm circuit.The loop relay R1 should have low-current consumption and besufficiently robust not to be operated by vibration.

For a combined intruder alarm fire or bandit alarm circuit (FIG. 3) thetwo relays R1, R2 are chosen to have a predetermined resistance/ampereturn relationship, and the ohmic value of resistors r to r will dependupon the characteristics of the relays R1, R2.

The system of the present invention provides complete 24 hour monitoringwhich implies that relay R1 (FIGS. 2 and 3) is continuously energized,being deenergized only when a detector is operated.

This condition is unsatisfactory since the relay may become stuck in theenergized condition i.e., an unsafe fault condition. When switching fromthe occupied" to the unoccupied condition, or vice versa, the relayshould preferably be deenergized and an indication of the conditionprovided. An unsafe relay condition would thus not escape detection.During the momentary period during which the relay was deenergized ontest, the alann devices would be switched out of circuit.

The present invention relates to fire protection circuits employingeither fire detection devices or fire alarm warning devices, or both.Fire detection devices include devices which detect the presence offire, heat or smoke where fire alarm warning devices are devices whichare manually operable push buttons operated to give a warning indicationof the existence of an outbreak of fire. The contacts F1 to F4 (FIG. 3)may include either or both types of device.

Although the series-connected contacts have been described withparticular application to doors, they may be the contacts of any form ofintruder detection device such as for instant invisible infrared rayunits ringing the exterior of a building or located within the building.

A manually operable control unit for use with the protective system ofthe present invention is described in copending British Pat. applicationNo. 51,843/68 in which switching mechanism is provided to enable thesystem to be switched from one operative mode to the other and includinglocking means for preventing the switchover to the second operative modewhenever a fault or alann condition prevails.

In a modification of the present invention, the loop comprises only twowires terminated by a resistor and no return to the control unit isrequired in such an embodiment. Current is arranged to flow through theloop to maintain a transistor relay in the OFF" condition when the loopis operative, i.e., giving complete protection. if the two wires of theloop are wholly or partly short-circuited, or one or both areopen-circuited", the transistor is rendered conductive to energize ordeenergize the winding of the relay and give an immediate alarm. Theterm transistor relay is intended to indicate at least one transistorconnected in circuit with an electromagnetic relay, but the latter mayif required be replaced by a power transistor or other solid-statedevice such as a siliconcontrolled rectifier.

Previously, two-wire loop systems have been of the open circuit type butthis embodiment of the invention has numerous advantages over presenttwo and four wire systems and in particular, two-channel operation isobtainable, as follows:

Preferably normally closed contacts are associated with doors, windowsand like points of entry and the contacts are arranged to be held openwhen the door or window is close. When a door is opened, the contact isclosed to effectively short circuit the protective loop. The diodes areconnected in series with the contacts and if a resistor is connected inseries with the diode, the short circuit current will be limited invalue. The value of the resistor is chosen such as to prevent transistorrelay from operating but a second transistor relay" is arranged tooperate to give second channel operation. By interlocking the tworelays, the second one is prevented from operating under short circuitconditions.

lclaim:

l. A protective system comprising a single electrical loop including abistate control device associated with an alarm circuit, said bistatecontrol device operatively changes from one state to another whenever anabnormal situation against which protection is required occurs, the loopbeing connected to a voltage source and also including a plurality ofdetection contacts actuated whenever such abnormal situation occurs; arespective unidirectional currentconducting device connected across atleast selected ones of said contacts such that the system is capable ofoperating in accordance with one current-conducting mode despiteactuation of any of said at least selected ones of said contacts andswitching means to inhibit current flow through the unidirectionalcurrent-conducting devices to permit the bistate control device tochange to the other of said operative states whenever any one of saidcontacts is actuated such that the system operates in accordance with asecond current-conducting mode and thereby provides continuousmonitoring of the loop.

2. A protective system comprising a single closed electrical loopincluding the winding of a relay having contacts associated with analarm circuit, the loop also including a plurality of series-connectedcontacts arranged to be actuated when an abnormal situation occurs, arespective unidirectional current-conducting device connected across atleast selected ones of said contacts such that the system is capable ofoperating in accordance with one current-conducting mode and switchingmeans to inhibit current flow through said unidirectionalcurrent-conducting devices such that the system operates in accordancewith a second current-conducting mode.

3. A protective system as claimed in claim 2 wherein saidseries-connected contacts are each normally closed and arranged to beopened whenever the presence of an intruder is detected, such as tochange the state of said relay, said unidirectional current conductingdevices each comprising a semiconductor diode connected across arespective seriesconnected contact, such that for said onecurrent-conducting mode, whenever a series-connnected contact is opened,current continues to flow around the loop to maintain the normal stateof said relay, and said switching means being arranged to reverse thedirection of current flow through the loop such that thereafter duringsaid second currentconducting mode, whenever a series-connected contactis opened, the current flow through the loop is interrupted to changethe state of said relay and provide an alarm indication of the presenceof an in truder.

4. A protective system as claimed in claim 2 wherein in ad dition tosaid series-connected contacts, at least one parallelconnected contactis provided with a unidirectional currentconducting device connected inseries therewith.

5. A protective system as claimed in claim 4 wherein theparallel-connected contacts are normally open and arranged to be closedwhenever the presence of an intruder is detected said at least oneunidirectional current-conducting device comprising a semiconductordiode connected in series with the associated parallel-connected contactsuch that whenever the contact closes during said one current-conductingmode, the reverse connection of said diode substantially inhibits thepassage of current therethrough and no short circuiting of the relaywinding occurs, said switching means being arranged to reverse thedirection of current flow through the loop such that thereafter, duringsaid second current-conducting mode, whenever a parallel-connectedcontact is closed, the diode passes current to decrease the flow ofcurrent through the winding of the relay and provide an alarm indicationof the presence of an intruder.

6. A protective system as claimed in claim 2 wherein an additional relayis included in the loop together with a plurality of additionalparallel-connected contacts connected in series with impedance meanssuch that when an additional parallelconnected contact is actuated, thecurrent flow through said additional relay is increased to an extentsufficient to change the state thereof but without effecting a change inthe state of said first-mentioned relay.

7. A protective system as claimed in claim 6 wherein said additionalparallel-connected contacts are normally-open and arranged to be closedwhenever heat, fire or smoke is detected and said additional relayhaving associated contacts connected in circuit with a fire warningcircuit such as to provide an alarm indication of the presence of saidfire, heat, or smoke.

8. A single closed electrical loop for the protective system of claim 2including a single relay winding connected in series with a DC voltagesource and a plurality of series-connected contacts each arranged toprovide intruder protection at a respective location and to be actuatedby an intruder and selected ones of said series-connected contacts eachhaving a unidirectional current-conducting device connected in paral leltherewith, said unidirectional current-conducting devices beingconnected such that when the current flows through the loop in onedirection opening of one of said selected ones of said series-connectedcontacts fails to interrupt the current flow through the winding of therelay and switching means for changing the direction of current flowthrough the loop whereby the unidirectional current-conducting devicesthereafter inhibit current flow through the winding of the relaywhenever an associated series-connected contact is opened.

9. A single closed electrical loop as claimed in claim 8 includingnormally-open parallel-connected contacts each arranged to provideintruder protection at a respective specific location and to be closedby an intruder and at least one of said parallel-connected contactshaving a unidirectional current-conducting device connected in seriestherewith and connected such that when the current flows through theloop in one direction closing of an associated contact fails tointerrupt the current flow through the winding of the relay, and whensaid switching means changes the direction of current flow through theloop closing of a contact thereafter interrupts current flow through thewinding of the relay.

10. A protective system as claimed in claim 1 wherein the loop includesswitching means for connecting the loop alternately to a DC and an ACvoltage source, the loop also including a plurality of series-connectedcontacts, selected ones of which having capacitor means connected inparallel therewith, and a relay having contacts associated with an alarmcircuit, the arrangement being such that when the loop is connected tothe AC source, the current through the loop is maintained at asufficient level to retain the winding of said relay energized despitethe actuation of any series-connected contact shunted by said capacitormeans, and when the loop is connected to said DC voltage source, thecurrent through the loop is interrupted in response to the actuation ofany seriesconnected contact, such as to change the state of said relay.

11. A protective system as claimed in claim 1 wherein the loop includesa plurality of series-connected contacts shunted by impedance means anda relay having contacts associated with an alarm circuit, the voltagelevel of the voltage source being sufficient to maintain the state ofthe relay despite actuation of any series-connected contact shunted byimpedance means and means for reducing the voltage such tat the voltageacross the relay is insufficient to maintain the stare thereof whenevera series-connected contact is actuated.

12. A protective system as claimed in claim 1 wherein the loop includesa plurality of series-connected contacts and a relay having associatedcontacts connected in an alarm circuit, and a capacitor connected acrosssaid relay winding, such that whenever a series-connected contact isactuated, the capacitor maintains the state of the relay, and switchingmeans for removing the capacitor from the loop such that actuation of aseries-connected contact effects a change of state of the relay.

13. A protective system as claimed in claim 2 wherein eachunidirectional current-conducting device is incorporated within a switchunit containing the respective contacts.

14. A protective system as claimed in claim 13 wherein the contacts areof the magnetic reed type.

15. A protective system as claimed in claim 2 wherein eachunidirectional current-conducting device is incorporated within a switchunit containing the respective contacts; the said unidirectional deviceand an impedance means being connected in series with or across thecontacts of the switch.

16. A protective system as claimed in claim 1 wherein a two wire loop isterminated by impedance means first relay means arranged to be actuatedwhenever a whole or partial short circuit or open circuit conditionoccurs in the loop, second relay means operable to provide secondchannel operation and interlocking means associated with the two relaymeans to prevent operation of the second relay means should such a wholeor partial short circuit or open circuit condition occur.

1. A protective system comprising a single electrical loop including abistate control device associated with an alarm circuit, said bistatecontrol device operatively changes from one state to another whenever anabnormal situation against which protection is required occurs, the loopbeing connected to a voltage source and also including a plurality ofdetection contacts actuated whenever such abnormal situation occurs; arespective unidirectional current-conducting device connected across atleast selected ones of said contacts such that the system is capable ofoperating in accordance with one currentconducting mode despiteactuation of any of said at least selected ones of said contacts andswitching means to inhibit current flow through the unidirectionalcurrent-conducting devices to permit the bistate control device tochange to the other of said operative states whenever any one of saidcontacts is actuated such that the system operates in accordance with asecond current-conducting mode and thereby provides continuousmonitoring of the loop.
 2. A protective system comprising a singleclosed electrical loop including the winding of a relay having contactsassociated with an alarm circuit, the loop also including a plurality ofseries-connected contacts arranged to be actuated when an abnormalsituation occurs, a respective unidirectional current-conducting deviceconnected across at least selected ones of said contacts such that thesystem is capable of operating in accordance with one current-conductingmode and switching means to inhibit current flow through saidunidirectional current-conducting devices such that the system operatesin accordance with a second current-conducting mode.
 3. A protectivesystem as claimed in claim 2 wherein said series-connected contacts areeach normally closed and arranged to be opened whenever the presence ofan intruder is detected, such as to change the state of said relay, saidunidirectional current conducting devices each comprising asemiconductor diode connected across a respective series-connectedcontact, such that for said one current-conducting mode, whenever aseries-connnected contact is opened, current continues to flow aroundthe loop to maintAin the normal state of said relay, and said switchingmeans being arranged to reverse the direction of current flow throughthe loop such that thereafter during said second current-conductingmode, whenever a series-connected contact is opened, the current flowthrough the loop is interrupted to change the state of said relay andprovide an alarm indication of the presence of an intruder.
 4. Aprotective system as claimed in claim 2 wherein in addition to saidseries-connected contacts, at least one parallel-connected contact isprovided with a unidirectional current-conducting device connected inseries therewith.
 5. A protective system as claimed in claim 4 whereinthe parallel-connected contacts are normally open and arranged to beclosed whenever the presence of an intruder is detected said at leastone unidirectional current-conducting device comprising a semiconductordiode connected in series with the associated parallel-connected contactsuch that whenever the contact closes during said one current-conductingmode, the reverse connection of said diode substantially inhibits thepassage of current therethrough and no short circuiting of the relaywinding occurs, said switching means being arranged to reverse thedirection of current flow through the loop such that thereafter, duringsaid second current-conducting mode, whenever a parallel-connectedcontact is closed, the diode passes current to decrease the flow ofcurrent through the winding of the relay and provide an alarm indicationof the presence of an intruder.
 6. A protective system as claimed inclaim 2 wherein an additional relay is included in the loop togetherwith a plurality of additional parallel-connected contacts connected inseries with impedance means such that when an additionalparallel-connected contact is actuated, the current flow through saidadditional relay is increased to an extent sufficient to change thestate thereof but without effecting a change in the state of saidfirst-mentioned relay.
 7. A protective system as claimed in claim 6wherein said additional parallel-connected contacts are normally-openand arranged to be closed whenever heat, fire or smoke is detected andsaid additional relay having associated contacts connected in circuitwith a fire warning circuit such as to provide an alarm indication ofthe presence of said fire, heat, or smoke.
 8. A single closed electricalloop for the protective system of claim 2 including a single relaywinding connected in series with a DC voltage source and a plurality ofseries-connected contacts each arranged to provide intruder protectionat a respective location and to be actuated by an intruder and selectedones of said series-connected contacts each having a unidirectionalcurrent-conducting device connected in parallel therewith, saidunidirectional current-conducting devices being connected such that whenthe current flows through the loop in one direction opening of one ofsaid selected ones of said series-connected contacts fails to interruptthe current flow through the winding of the relay and switching meansfor changing the direction of current flow through the loop whereby theunidirectional current-conducting devices thereafter inhibit currentflow through the winding of the relay whenever an associatedseries-connected contact is opened.
 9. A single closed electrical loopas claimed in claim 8 including normally-open parallel-connectedcontacts each arranged to provide intruder protection at a respectivespecific location and to be closed by an intruder and at least one ofsaid parallel-connected contacts having a unidirectionalcurrent-conducting device connected in series therewith and connectedsuch that when the current flows through the loop in one directionclosing of an associated contact fails to interrupt the current flowthrough the winding of the relay, and when said switching means changesthe direction of current flow through the loop closing of a contactthereafter interrupts current flow tHrough the winding of the relay. 10.A protective system as claimed in claim 1 wherein the loop includesswitching means for connecting the loop alternately to a DC and an ACvoltage source, the loop also including a plurality of series-connectedcontacts, selected ones of which having capacitor means connected inparallel therewith, and a relay having contacts associated with an alarmcircuit, the arrangement being such that when the loop is connected tothe AC source, the current through the loop is maintained at asufficient level to retain the winding of said relay energized despitethe actuation of any series-connected contact shunted by said capacitormeans, and when the loop is connected to said DC voltage source, thecurrent through the loop is interrupted in response to the actuation ofany series-connected contact, such as to change the state of said relay.11. A protective system as claimed in claim 1 wherein the loop includesa plurality of series-connected contacts shunted by impedance means anda relay having contacts associated with an alarm circuit, the voltagelevel of the voltage source being sufficient to maintain the state ofthe relay despite actuation of any series-connected contact shunted byimpedance means and means for reducing the voltage such that the voltageacross the relay is insufficient to maintain the stare thereof whenevera series-connected contact is actuated.
 12. A protective system asclaimed in claim 1 wherein the loop includes a plurality ofseries-connected contacts and a relay having associated contactsconnected in an alarm circuit, and a capacitor connected across saidrelay winding, such that whenever a series-connected contact isactuated, the capacitor maintains the state of the relay, and switchingmeans for removing the capacitor from the loop such that actuation of aseries-connected contact effects a change of state of the relay.
 13. Aprotective system as claimed in claim 2 wherein each unidirectionalcurrent-conducting device is incorporated within a switch unitcontaining the respective contacts.
 14. A protective system as claimedin claim 13 wherein the contacts are of the magnetic reed type.
 15. Aprotective system as claimed in claim 2 wherein each unidirectionalcurrent-conducting device is incorporated within a switch unitcontaining the respective contacts; the said unidirectional device andan impedance means being connected in series with or across the contactsof the switch.
 16. A protective system as claimed in claim 1 wherein atwo wire loop is terminated by impedance means first relay meansarranged to be actuated whenever a whole or partial short circuit oropen circuit condition occurs in the loop, second relay means operableto provide second channel operation and interlocking means associatedwith the two relay means to prevent operation of the second relay meansshould such a whole or partial short circuit or open circuit conditionoccur.